Q&A

When the car won't start put a voltmeter across the battery and check its voltage it should be 12.6 volts at rest. It could be a bad or unsecured battery ground cable between the battery and chassis or chassis and engine. Turn on the ignition switch to the on but not run position do you see an icon on the instrument panel representing a battery, if you do not see this battery icon the alternator is not seeing power to allow it to charge. Start the engine does the battery icon go out. A more complex but good diagnostic test is checking "voltage drop" on the battery ground and power cables. You can find a video on how to do a voltage drop test on youtube. Put a volt meter across the battery terminals with the engine off, at rest the battery voltage should be 12.6 volts if the battery is fully charged. Now get a friend to start the car while you watch the voltmeter, the voltage reading across the battery should read approximately 14.6 volts if the alternator is charging correctly.
You may have some sort of battery draw that needs to be isolated and illuminated. You would need an amp meter disconnect one cable of the battery put your Amp meter in series with the battery cable that has been removed. Note the draw say for example 15 Amps, with the keys out of the ignition, doors closed driver's window down, start removing the fuse links and fuses one at a time until the voltage draw goes away to zero (or near zero). It is that circuit that is causing the electrical draw, then look at a wiring diagram, see what consumers are on that circuit and start to isolate each circuit to see which is the culprit. Modern cars have lots of computers and electronics that need constant battery even with the key off so parasitic draw of .2 Amps may be normal.